Leadframe designs for plastic overmold packages

ABSTRACT

The specification describes a plastic overmolded package for high power devices that has a very low lead count, typically fewer than eight, and in a preferred embodiment, only two. The leads occupy essentially the same linear space as the multiple leads in a conventional package and thus have a wide-blade configuration. To improve mechanical integrity, the leads in the package are provided with retention slots to add back the equivalent of the plastic joints in the spaces that were eliminated due to the wide-blade design. The retention slots extend in the width dimension of the leads.

FIELD OF THE INVENTION

This invention relates to leadframe designs used in plastic overmoldedpackages for integrated circuit (IC) and related devices.

BACKGROUND OF THE INVENTION

The most common form of packaging for electronic devices such as ICdevices is a plastic housing. Typically, the electronic components areassembled on a metal leadframe and a polymer is molded over the assemblyto encapsulate the device. The leadframe serves not only to support theelectronic components, but has metal tabs that extend from theovermolded plastic and provide a means to electrically connect to theencapsulated electronic components.

The leadframe, prior to assembly and encapsulation, typically is squareor rectangular and has a center paddle to which a semiconductor chip isdie bonded. The leads that provide electrical interconnection extendfrom the sides of the paddle, usually along two opposing edges of theleadframe, as in the common dual-in-line package (DIP). In highpin-count packages, such as quad flat packs, leads may extend from allfour sides.

In plastic overmolded packages with many leads per side, the plasticflows around the leads and forms a very integral structure. A leadframewith 10 leads per side for example provides 9 spaces where the plasticjoinswever, If the same space is occupied by only three leads forexample, only two spaces where the plastic joins are provided. Thus asthe number of leads are reduced, the mechanical integrity of the packagemay become an issue.

BRIEF STATEMENT OF THE INVENTION

We have designed a plastic overmolded package for high power devicesthat has a very low lead count, typically fewer than eight, and in apreferred embodiment, only two. The leads occupy essentially the samelinear space as the multiple leads in a conventional package and thushave a wide-blade configuration. To improve mechanical integrity, theleads in the package are provided with retention slots to add back theequivalent of the plastic joints in the spaces that were eliminated dueto the wide-blade design. The retention slots extend in the widthdimension of the leads.

BRIEF DESCRIPTION OF THE DRAWING

The invention may be better understood when considered in conjunctionwith the drawing in which:

FIG. 1 is a side view of a conventional twenty lead plastic overmoldedpackage;

FIG. 2 is a plan view of the package of FIG. 1;

FIG. 3 is a side view of a twin lead plastic overmolded package;

FIG. 4 is a plan view of a the plastic overmolded package of FIG. 3;

FIG. 5 is a side view of a modified plastic overmolded package havingsix leads, with three shown;

FIG. 6 is a plan view of the six lead plastic overmolded package of FIG.5;

FIG. 7 is a plan view of the twin lead plastic overmolded package ofFIG. 4 with a schematic showing of the internal portion of the leadframe;

FIG. 8 is an illustration of the problem of mechanical distortion orseparation of the leads in the twin lead plastic overmolded package;

FIG. 9 is one embodiment of a plastic overmolded twin lead package wherethe leads are provided with retention slots according to the invention;

FIG. 10 shows a six lead embodiment according to the invention;

FIGS. 11-13 are diagrams showing dimensions relevant to the design ofthe retention slots of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a conventional twenty lead plastic overmolded package. Thisfigure is nearly a scale representation of the MO-166 standard formatapproved by JEDEC. It should be understood at the outset that the MO-166packaging format is represented here by way of example only. A widevariety of plastic overmolded package designs are available in thepackaging industry. Many will share the properties described below. Allplastic overmolded package products that have the features to bedescribed are intended to be included in the scope of the invention.Referring again to FIG. 1, the plastic overmolded package body is shownat 11 and the twenty leads, ten to a side, are indicated at 12. In theembodiment shown, the leads are bent downward in a so-called gull-wingshape to allow for convenient surface mounting. This is the mostfamiliar JEDEC MO-166 plastic overmolded package profile. For a betterunderstanding, the plastic overmolded package of FIG. 1 is shown in FIG.2 in plan view. This type of package is typically installed, in thefinal assembly, on a printed wiring board, or equivalent, and theplastic overmolded package is attached to the printed wiring board usingsurface mount assembly techniques. In a surface mount arrangement thereusually are not significant forces that pull the leads (laterally) awayfrom the plastic overmolded body. This aspect of the mountingarrangement may be recalled in the discussion below, which contrasts afailure mode of the packages with the modified design of the inventionwith conventional multiple lead packages.

Plastic overmolded packages of the kind just described are used toencapsulate a variety of devices. We have modified the conventionalpackage in significant ways to accommodate a high power diode. Theparticular power device for which the plastic overmolded package designhas been modified has, in an electrical sense, two leads. The simplestembodiment is a plastic overmolded package wherein the leads in thestandard MO-166 configuration have been merged to a single lead perside, thus producing a twin lead plastic overmolded package. This designis shown in FIG. 3, where the single lead per side is shown at 31. Inthe particular plastic overmolded package design of FIG. 3, the leadsare so-called bayonet leads. They are tab-like in form, and are not bentfor mounting. A plan view for this embodiment is shown in FIG. 4. Aconsequence of this design, where tab-like leads are used, is thatlateral forces that tend to pull the tab away from the plastic overmold,or bend the tab thus distorting the package, are not uncommon. This isdescribed in more detail below.

Although the power diode described here requires only two electricalinterconnections, it is not unusual for a single electrical lead to besplit. Accordingly, the single lead per side shown in FIGS. 3 and 4 maybe split into two or more sections. For example, the embodiment of FIG.5 shows the single lead (per side) of the plastic overmolded package ofFIGS. 3 and 4 split to three leads 51 per side. This plastic overmoldedpackage design is shown in plan view in FIG. 6.

FIGS. 1-6 illustrate the plastic overmolded package outline, i.e. theoutside form of the package. The leads that extend from the package aretypically part of a leadframe. In well-known fashion, the lead frameorganizes the lead configuration that extends from the edges of theplastic overmolded package. It also provides a platform for the deviceor devices that are encapsulated inside the plastic overmold. This isillustrated in a schematic way in FIG. 7, where the portion of theleadframe that is housed within the plastic encapsulant is shownoutlined in phantom at 71. The device, in this example a power diode, isshown at 73. The power diode is a typically a silicon chip that iselectrically connected to the leads 71. Alternatives exist for mountingand connecting the chip 73. The leadframe may contain a center paddle 72that supports the chip. The chip may then be connected to the leads withwire bonds or other suitable means. Alternatively, the chip may be bumpbonded using solder bumps to each of the leads 71.

Merging the twenty leads of the plastic overmolded package of FIGS. 1and 2 into two, or even six may affect the mechanical performance of theplastic overmolded package. A typical plastic overmolded package ismolded in a molding operation that produces two plastic masses that arejoined, while still uncured, to form the shape shown in FIG. 1 forexample. This produces a seam, indicated at 14, where the two uncuredplastic bodies join together. The seam occurs also between each of theleads 12, thus producing a molded structure with a finite integrity thatis partly attributable to the multiple seams between the leads. It isintuitively easy to understand that the in the modified designs of FIGS.3 and 5, much, or all, of the portion of the seam between the leads iseliminated. This may have implications for the mechanical integrity ofthe plastic overmolded package. In particular, the tendency of the leadsto pull away from the plastic overmold is increased. This is due to atleast two physical features. One, the tab-like design, and the mountingtechnique, are more likely to result in significant lateral forces onthe leads. And two, merging many leads into several, or two, multipliesthe lateral force per lead.

The problem just outlined is illustrated in FIG. 8 where lead 81 isshown displaced from the intended position due to one or a combinationof physical forces on the single lead. Obviously if the displacement islarge enough to cause a separation from the device 73, that will causean electrical failure. However, even smaller displacements, in factrelatively small strains, may fracture a solder bond, or damage a wirebond, and cause failure.

To increase the mechanical integrity of the plastic overmolded packagewe have modified the design of the leadframe. FIG. 9 shows oneembodiment of the invention wherein slots 91 are provided through theleadframe prior to overmolding. This forms a trough through which theplastic flows. and joins from one side of the mold to the other. Thisrestores the portions of the seam that was discussed earlier. The seamin this case is still absent from the outer or visible edge, but existswithin the overmold.

An alternative embodiment is shown in FIG. 10 where there are threeslots 92 provided in each tab lead 31. It can be concluded that anynumber of slots may be used. Moreover, they may be side-by-side as shownin these figures, or formed in one or more rows. However, due to spaceconsiderations the side-by-side configuration will usually be preferred.

To more completely appreciate the mechanical consequences of the slots91 in FIG. 9, or 92 in FIG. 10, a discussion of the nature of theretention forces on the twin leads 31 may be helpful. Although, asmentioned above, it is intuitively easy to understand that the retentionforce for lead 31 is improved in the design of FIG. 9, the actual forceanalysis is quite complex.

Some of the relevant force considerations are represented in FIG. 11. Itshould be understood that there is no attempt here to present a rigorousforce analysis but only to illustrate to first order some of the mostrelevant force aspects of the retention slots of the invention. In FIG.11, the pull force on the lead is represented by the arrow f_(p) and theretention forces by arrow f_(r). In the tab lead structure of FIGS. 3and 5, the main retaining forces for the tab lead shown are adhesiveforces between the tab surface and the polymer surface. Force at isalong the inside end of the tab lead and is a polymer to metal adhesiveforce in tension. Force as is along the side edges and the top andbottom of the tab lead and is a polymer to metal adhesive force inshear. In each case the force is a function of the area. The polymer totab adhesive force in tension, a_(t) is small compared with a_(s). Thisis because polymer to metal adhesion in tension is less than in shear,and because the area for at is small compared with the area for a_(s).Thus the f_(r) component attributable to a_(t) may be disregarded, andforce f_(r) is approximated as equal to twice area of the tab in thehorizontal plane x a_(s).

The corresponding force diagram with the slot configuration is shown inFIG. 12. The tab lead is shown with a slot at 93 where the plastic flowscontinuously through the slot from the top plastic mass to join thebottom plastic mass. Now the retention force picture changesdrastically. The adhesive retention forces a_(t) and a_(s) are stillpresent, although the area of the as force is reduced by the area of theslot. Also a_(t) is essentially doubled, but still regarded asinconsequential. A new force is added, represented by p, which is apolymer fracture force for the polymer molding material. To polymerfracture force is very large compared with any of the adhesive forces.If an assumption is made that the polymer is relatively rigid, which isqualitatively the case for thermoset molding compounds, the force p isthe polymer bond fracture force times the area of the edge of the slotindicated at p. FIG. 13 shows slot 93 in plan view. It is evident thatthe retention forces attributed to adhesion at the sides of the slot,indicated at as, and the edge designated at, are small. The mainretention force is the retention force associated with the wall of theslot designated p, where the retention force equation is dominated bythe polymer fracture force. This force is a direct function of lengthL_(p).

From the brief analysis above, it is evident that the portion of theslot indicated at a_(t) is relatively inconsequential. This means, interalia, that the width of the slot is immaterial as long as it is wideenough for the polymer to flow through from each side and join. Theability of the polymer to flow through the slot is also a function ofthe aspect ratio of the slot. It is recommended that the aspect ratio beat least 1 to ensure flow through. Stated otherwise, the width of theslot should be at least equal to the thickness of the tab lead. Themaximum width of the slot, as just stated, is inconsequential, and canbe as wide as desired without limitation. (A caution, the slot widthshould not be confused with the lead width. They are not along the sameaxis.) Since the edge of the slot that is outboard with respect to theplastic overmolded package (indicated at E_(O) in FIG. 13) provideslittle retaining force, it is not important, from the standpoint ofmeeting the goals of the invention, for the outboard side to lie withinthe plastic overmold. The inboard edge, E_(i) in FIG. 13, is desirablymaximixed in L_(p).

A similar analysis can be done with the embodiment of FIG. 5. In thisdesign, dimension L_(p), the dominant determinant of f_(r) is somewhatreduced. However, it is still significant overall. In terms of meetingthe goals of the invention, it is considered adequate if the integratedlength L_(p) (the sum of the slot lengths in FIG. 5) is greater than 30%of the total width of the lead. In the usual case this prescription is:L _(p)>0.3[(N×w)+(N−1)s]

-   -   where L_(p) is the sum of the slot lengths on a side of the        plastic overmolded package, N is the number of leads on a side,        w is the width of the leads, and s is the linear separation        between the leads. As mentioned above, in the preferred case        N=1-3.

The foregoing analysis shows that by far the most effective feature fora retaining tab lead against the pull forces described is an opening inthe tab that has a maximum wall length normal to the direction of thepull force. From this it can be deduced that a round opening is lesseffective. The force retention performance of the edge of a round holetransitions from pure a_(s) to p. Moreover, the space on the leadframe,and the space internal of the package, that may be devoted to aretention feature is limited. That space is usually rectangular with alarge aspect ratio, i.e. large ratio of length L_(s) to width W_(s).This follows from the usual circumstance that the chip occupies most ofthe area of the plastic overmolded package. Typically there is a spacealong the edges of the chip where retention features may be placed, butthat space is limited. A slot with an essentially straight inboard edgeis an efficient structure in terms of maximizing L_(P).

The analysis set forth above assumes a lateral pull force. Other forcesare encountered in the use environment that also degrade the mechanicaland electrical performance of the plastic overmolded package. Forexample, twisting of the leads is common. From a qualitativeperspective, the retention slots of the invention are useful forimproving the overall mechanical integrity of the plastic overmoldedpackage, and mitigate against most types of mechanical strains anddeformations that the device will see in use.

As mentioned above, the embodiments shown are examples of plasticovermolded packages with leads extending from the edges of the plasticovermold. The packages generally have a quadrilateral shape. Theleadframes within the plastic overmold typically have a center dieattachment region where the die is bonded, and leads extending from twoopposing sides. The invention may also be applied to a quad pack designwhere leads extend from four sides.

A slot by definition has an aspect ratio of length to width greater than1, usually significantly greater than 1. The edges along the length,i.e. the longest edges are referred to below as the major edges. In thedevice structures shown here one of the major edges is closest to thecenter of the plastic overmolded package, and is referred to below asthe inboard major edge. The other edge, furthest from center of thepackage, is referred to as the outboard edge. The edges of the slot aretypically essentially straight. However, it is recognized that theoutboard major edge is relatively incidental to the goals of theinvention and thus may assume any reasonable shape. Moreover, the aspectratio of the slot assumes less importance due to the minimalconsequences of the outboard edge.

Various additional modifications of this invention will occur to thoseskilled in the art. All deviations from the specific teachings of thisspecification that basically rely on the principles and theirequivalents through which the art has been advanced are properlyconsidered within the scope of the invention as described and claimed.

1. A plastic overmolded package comprising: a. a leadframe, theleadframe comprising: i. a center die attachment portion, ii. at leasttwo tab leads extending from the center die attachment portion, the tableads having a width W and a thickness T, iii. at least one slot formedin each of the tab leads, the slot extending through the thickness T ofthe tab leads and having an inboard edge E_(i) nearest to the device andan outboard edge furthest from the device, b. a semiconductor deviceattached to the leadframe, c. a plastic overmold having a top, a bottomand sidewalls encapsulating: the semiconductor device, the center dieattachment portion of the leadframe, and the inboard edge E₁ of the slotin the tab leads, while leaving a substantial portion of the tab leadsexposed, the exposed portions of the tab leads extending away from thesidewalls of the overmold.
 2. The package of claim 1 wherein the inboardedge is essentially straight.
 3. The package of claim 2 wherein theleadframe comprises two tab leads.
 4. The package of claim 1 wherein theleadframe comprises two to six tab leads.
 5. The package of claim 3wherein the inboard edge has length L_(P) and L_(P) is greater than0.3×W.
 6. The package of claim 1 wherein the slot has width w_(s) andthe tab lead has thickness t_(l) and w_(s)/t_(l) is at least
 1. 7. Thepackage of claim 4 wherein the leadframe comprises 4 to 6 leads, with 2or 3 leads per side, and the leads separated by distance s, wherein:L_(p)>0.3 [(N×w _(l))+(N−1)s] where L_(p) is the sum of the inboardedges on a side of the plastic overmolded package, N is the number ofleads on a side, and w_(l) is the width of the leads.